Important changes:
* Most functions are now methods associated to classes
* The methods in `accessionid` and `taxid` now take as arguments: the database type and either a SQLite, MySQL or Postgres database followed by the necessary arguments depending of the database type
* The `est`, `gb`, `gss`, `wgs` and `prot` tables have been merged in one `accession` table
* A chunk size limit has been added (prevents running into `too many SQL variables` )